1. Field of the Invention
The present invention relates to a photoelectric converting circuit including a photoelectric converter element and an integrating circuit using an operational amplifier.
2. Description of Related Art
Conventional photoelectric converting circuits employing a photoelectric converter element such as a photodiode are shown in FIGS. 1A-1C.
In FIG. 1A, a conventional photoelectric converting circuit comprises a photodiode 1, a read switch 2, a reset switch 3, and a load capacitance 4. In FIG. 1B, another conventional photoelectric converting circuit comprises a source follower FET 5 that amplifies the output of the photodiode 1. The source follower FET 5 contains a parasitic capacitance C.sub.1.
The photoelectric converting circuit of FIG. 1A stores the photoelectric current generated by the photodiode into the load capacitance 4 in a fixed time. The voltage .DELTA.V output from the load capacitance 4 is expressed as follows: ##EQU1## where C.sub.PD is the junction capacitance of the photodiode 1, C.sub.L is the capacitance of the load capacitance 4, and i is the photoelectric current generated by the photodiode 1.
Similarly, the output voltage .DELTA.V of the photoelectric converting circuit of FIG. 1B is given by the following equation. ##EQU2##
The sensitivity of the photoelectric converting circuits of FIGS. 1A and 1B can be increased by reducing the junction capacitance C.sub.PD of the photodiode 1 or the parasitic capacitance C1 of the source follower FET 5. However, since the junction capacitance C.sub.PD of the photodiode 1 is directly proportional to the junction area of the photodiode 1, reducing the junction capacitance will also reduce the photoelectric current i. For this reason, it is difficult to reduce the junction capacitance C.sub.PD.
Likewise, the parasitic capacitance C.sub.1 of FIG. 1B cannot be reduced to an extremely small value in order to maintain the accuracy of the source follower FET 5 at a certain level.
To further improve the sensitivity, another photoelectric converting circuit using an operational amplifier as shown in FIG. 1C has been proposed.
In FIG. 1C, the reference numeral 6 designates an operational amplifier, the reference numeral 7 denotes an integration capacitance, and the reference numeral 8 designates a reset switch.
In the photoelectric converting circuit of FIG. 1C, the photoelectric current generated by the photodiode 1 is input to the integrating circuit composed of the operational amplifier 6 and the integration capacitance 7, stored in the integration capacitance 7, and amplified and output as the output voltage .DELTA.V.
During this operation, the inverting input terminal of the operational amplifier 6, to which the anode of the photodiode 1 is connected, is maintained at Vref. Accordingly, the photoelectric current i generated by the photodiode 1 is stored only into the integration capacitance 7 without being stored in the junction capacitance C.sub.PD and the parasitic capacitance C.sub.1. Therefore, the output .DELTA.V of the circuit is expressed by the following equation. ##EQU3## where C.sub.2 is the capacitance of the integration capacitance 7. The capacitance C.sub.2 can be changed independently because it is not associated with other characteristics as the capacitance C.sub.PD or C.sub.1. Thus, the sensitivity of the circuit can be increased by reducing the capacitance C.sub.2 because this will increase the output .DELTA.V.
The circuit of FIG. 1C, however, presents the following problem.
Reducing the capacitance C.sub.2 to improve the sensitivity causes a problem in that the effect of switching noise induced by turning off the reset switch 8 will increase. The reset switch 8 is usually composed of a MOSFET which includes a coupling capacitance C.sub.GD across the gate and drain, and a coupling capacitance C.sub.GS across the gate and source as shown in FIG. 2. Since the changes in the gate potential have an influence on the source and drain through these capacitances, the switching noise is applied to the input of the operational amplifier 6 in FIG. 1C when operating the reset switch 8.
The switching noise is amplified by the operational amplifier 6 whose amplification factor is substantially proportional to the reciprocal of the capacitance C.sub.2 of the integration capacitance 7. Therefore, the effect of the noise increases as the capacitance C.sub.2 reduces.
In addition, reducing the capacitance C.sub.2 will increase the effect of fabrication error. Specifically, fabricating a very small capacitance C.sub.2 requires a fine pattern to be formed, and the fabrication error increases as the pattern becomes finer.
As a result, a circuit including a plurality of sensors of FIG. 1C presents a problem in that the sensitivity varies from sensor to sensor because of the variation in the capacitance C.sub.2.
In summary, reducing the integration capacitance of the conventional photoelectric converting circuit using an operational amplifier to improve the sensitivity presents a problem in that the reduction in the integration capacitance will increase the output noise or the variation in sensitivity, thus limiting the increase in the sensitivity.